Method of controlling a surgical system based on irrigation flow

ABSTRACT

A surgical system that is able to sense the onset of an occlusion or other surgical event as well as the instant an occlusion breaks. To help avoid overheating of the tip, the system determines a temperature of an eye using irrigation flow rate and reduces the power to the handpiece automatically if an overheating situation is predicted. Alternatively or in addition, the system monitors the power drawn by the handpiece, which is indicative of the cutting load on the tip, and automatically adjusts the power or stroke of the tip to compensate for increased loads on the tip.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 10/818,314, filed Apr. 5, 2004, priority to whichis claimed under 35 U.S.C. §120, which claims priority to U.S.Provisional Application Ser. No. 60/555,240, filed Mar. 22, 2004, under35 U.S.C. §119. This application also claims priority under 35 U.S.C.§119 to U.S. Provisional Application Ser. No. 60/587,693, filed Jul. 14,2004.

FIELD OF THE INVENTION

The present invention relates generally to the field of ophthalmicsurgery and, more particularly, to a method of controlling surgicalparameters of a phacoemulsification system based on determinationsinvolving irrigation pressure.

BACKGROUND

The human eye functions to provide vision by transmitting light througha clear outer portion called the cornea, and focusing the image by wayof the lens onto the retina. The quality of the focused image depends onmany factors including the size and shape of the eye, and thetransparency of the cornea and lens. When age or disease causes the lensto become less transparent, vision deteriorates because of thediminished light which can be transmitted to the retina. This deficiencyis medically known as a cataract. An accepted treatment for cataracts isto surgically remove the cataract and replace the lens with anartificial intraocular lens (IOL). In the United States, the majority ofcataractous lenses are removed using a surgical technique calledphacoemulsification. During this procedure, a thin cutting tip isinserted into the diseased lens and vibrated ultrasonically. Thevibrating cutting tip liquefies or emulsifies the lens so that the lensmay be aspirated out of the eye. The diseased lens, once removed, isreplaced by an IOL.

A typical ultrasonic surgical device suitable for an ophthalmicprocedure includes an ultrasonically driven handpiece, an attachedcutting tip, an irrigating sleeve and an electronic control console. Thehandpiece assembly is attached to the control console by an electriccable or connector and flexible tubings. A surgeon controls the amountof ultrasound power that is delivered to the cutting tip of thehandpiece and applied to tissue at any given time by pressing a footpedal to request power up to the maximum amount of power set on theconsole. Flexible tubings supply irrigation fluid to and draw aspirationfluid from the eye through the handpiece assembly.

The operative part of the handpiece is a centrally located, hollowresonating bar or horn that is attached to a set of piezoelectriccrystals. The crystals are controlled by the console and supplyultrasonic vibrations that drive both the horn and the attached cuttingtip during phacoemulsification. The crystal/horn assembly is suspendedwithin the hollow body or shell of the handpiece by flexible mountings.The handpiece body terminates in a reduced diameter portion or noseconeat the body's distal end. The nosecone is externally threaded to acceptthe irrigation sleeve. Likewise, the horn bore is internally threaded atits distal end to receive the external threads of the cutting tip. Theirrigation sleeve also has an internally threaded bore that is screwedonto the external threads of the nosecone. The cutting tip is adjustedso that the tip projects only a predetermined amount past the open endof the irrigating sleeve.

In use, the ends of the cutting tip and the irrigating sleeve areinserted into a small incision of predetermined width in the cornea,sclera, or other location. One known cutting tip is ultrasonicallyvibrated along its longitudinal axis within the irrigating sleeve by thecrystal-driven ultrasonic horn, thereby emulsifying the selected tissuein situ. The hollow bore of the cutting tip communicates with the borein the horn that in turn communicates with the aspiration line from thehandpiece to the console. Other suitable cutting tips includepiezoelectric elements that produce both longitudinal and torsionaloscillations. One example of such a cutting tip is described in U.S.Pat. No. 6,402,769 (Boukhny), the contents of which are incorporatedherein by reference.

A reduced pressure or vacuum source in the console draws or aspiratesthe emulsified tissue from the eye through the open end of the cuttingtip, the cutting tip and horn bores and the aspiration line, and into acollection device. The aspiration of emulsified tissue is aided by asaline solution or other irrigant that is injected into the surgicalsite through the small annular gap between the inside surface of theirrigating sleeve and the cutting tip.

One known surgical technique is to make the incision into the anteriorchamber of the eye as small as possible in order to reduce the risk ofinduced astigmatism. These small incisions result in very tight woundsthat squeeze the irrigating sleeve tightly against the vibrating tip.Friction between the irrigating sleeve and the vibrating tip generatesheat. The risk of the tip overheating and burning tissue is reduced bythe cooling effect of the aspirated fluid flowing inside the tip.

When the tip becomes occluded or clogged with emulsified tissue, theaspiration flow can be reduced or eliminated, allowing the tip to heatup, thereby reducing cooling and resulting in temperature increase,which may burn the tissue at the incision. In addition, duringocclusion, a larger vacuum can build up in the aspiration tubing so thatwhen the occlusion eventually breaks, a larger amount of fluid can bequickly suctioned from the eye, possibly resulting in the globecollapsing or other damage to the eye.

Known devices have used sensors that detect large rises in aspirationvacuum, and detect occlusions based a particular predetermined vacuumlevel. Based on this sensed occlusion, power to the handpiece may bereduced and/or irrigation and aspiration flows can be increased. SeeU.S. Pat. Nos. 5,591,127, 5,700,240 and 5,766,146 (Barwick, Jr., etal.), the entire contents of which are incorporated herein by reference.These devices, however, use a fixed aspiration vacuum level to trigger aresponse from the system. This fixed level is a threshold value basedupon a fixed percentage of the selected upper vacuum limit. The use andeffectiveness of such systems, however, are limited since they do notrespond until that preset vacuum level is reached. U.S. Pat. No.6,179,808 to Boukhny, et. al., the entire contents of which areincorporated herein by reference, describes a system that reduces signalamplitude and/or duty cycle when the temperature exceeds a predeterminedlimit, as calculated based on the measured or estimated irrigation flow.

Known occlusion sensing systems can thus be improved since, in reality,vacuum levels can vary over a short period of time during differentstages of occlusion. Setting this preset vacuum limit too low results inthe system changing its operating parameters prematurely, and holding onto those parameters after the occlusion has cleared. Setting the limittoo high can result in the system changing its setting too close to theactual occurrence of the occlusion, and changing its setting back tonormal prior to the clearance of the occlusion. In addition, cuttingefficiency is maximized when the cutting tip is occluded, so increasingpower when an occluded condition is detected maximizes cuttingefficiency, but increases the risk of overheating the tissue surroundingthe tip.

Further, throughout the surgery, there are times when the tip ispressing against the lens in order to emulsify lens tissue, and thereare times when the tip is not in contact with the lens. Ultrasoundenergy, however, remains on until the surgeon releases the foot pedal,even during times when the lens material is aspirated, the surgeon pullsthe tip away from the lens, or the lens moves away from the tip. Theefficiency of the surgery decreases, and the wasted energy can causeunnecessary heating of the tip, which may increase the likelihood of anundesirable burn to the tissue at the incision.

Therefore, a need continues to exist for an occlusion detection systemthat more accurately detects the occurrence and clearance of anocclusion in a surgical aspiration system. This information can be usedby the control system to adjust power accordingly, e.g., increasingpower during an occlusion in order to improve the cutting efficiency ofthe ultrasound tip and/or reducing power when the relative temperaturereaches a predetermined threshold in order to prevent excessive heating.Cutting efficiency may be further increased by adding a load detectionsystem that detects when the tip is no longer in contact with lensmaterial and adjusts power automatically.

SUMMARY

In accordance with one embodiment is a method of controlling a surgicalsystem by establishing a threshold temperature and monitoring anirrigation pressure in an irrigation line running to an ultrasoundhandpiece. A rate of flow of irrigation fluid and a heat absorptioncapacity for the flow of irrigation fluid are calculated. A temperatureof the eye is determined by comparing or analyzing the heat absorptioncapacity and the power supplied to the ultrasound handpiece to determinea temperature of an eye, which is compared to the threshold temperature.The amount of power that is delivered to a cutting tip of an ultrasoundhandpiece is adjusted as necessary based on the comparison.

According to another embodiment, a threshold temperature is establishedand an irrigation pressure of the ultrasound handpiece is monitored. Therate of irrigation fluid flow is calculated and a heat absorptioncapacity for the irrigation fluid flow rate is determined. A temperatureof an eye is determined based on the heat absorption capacity and powersupplied to an ultrasound handpiece. The eye temperature is compared tothe threshold temperature. In addition, a rate of change of irrigationpressure is determined, and a stage of occlusion is determined based onthe rate of change. The power delivered to a cutting tip of anultrasound handpiece is adjusted as necessary based on the stage ofocclusion and the comparison.

In a further alternative embodiment, a method of controlling a surgicalsystem includes establishing a threshold temperature, monitoring anirrigation pressure and calculating a rate of flow of the irrigationfluid, a temperature of an eye is determined based on the heatabsorption capacity for the calculated rate of flow of irrigation fluidand power supplied to an ultrasound handpiece. The eye temperature iscompared to the threshold temperature. In addition, the rate of changeof the irrigation pressure is determined. A stage of occlusion isdetermined based on the rate of change and the power delivered to acutting tip of an ultrasound handpiece is adjusted as necessary based onthe stage of occlusion and the comparison. For example, the powerdelivered to the handpiece can be reduced if the determined temperatureexceeds the threshold temperature, whereas power can be maintained orincreased if the determined temperature is below the thresholdtemperature.

According to various embodiments, the power can be adjusted by adjustinga duty cycle or adjusting a stroke or amplitude of the handpiece outputof the ultrasound handpiece. The stages of occlusion that can beidentified include an onset or preliminary indication of occlusion, apre-occlusion condition, a full occlusion, an occlusion break or arecovery from a break of an occlusion.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, in which like reference numbers representcorresponding parts throughout and in which:

FIG. 1 is a perspective view of an exemplary surgical system that may beused with various embodiments;

FIG. 2 is block diagram showing components of a surgical system;

FIGS. 3A-B illustrate exemplary ultrasonic handpieces that can be usedwith various embodiments;

FIG. 4 is a flow diagram illustrating one embodiment of a method ofadjusting power delivered to a handpiece power based on a pattern and arate of change of one or more operating parameters;

FIG. 5 illustrates the stages of occlusion that can be used with theembodiments of the present invention;

FIG. 6 illustrates patterns of aspiration vacuum and irrigation pressureat different stages of an occlusion shown in FIG. 5;

FIG. 7 is a flow diagram illustrating one embodiment of a method foradjusting power to prevent overheating of a transducer tip;

FIG. 8 is a more detailed flow diagram of one implementation of theembodiment shown in FIG. 7;

FIGS. 9 a-9 b illustrate exemplary burst mode pulses having constantamplitudes and different off times, and different off times beingcontrolled by depression of a foot pedal;

FIG. 10 illustrates exemplary pulse mode pulses having different dutycycles, and duty cycles being controlled by depression of a foot pedal;

FIG. 11 illustrates non-zero Sense Power levels between cutting pulsesaccording to one embodiment;

FIG. 12 flow diagram illustrating a method for adjusting power based onpower, threshold and sensitivity calculations;

FIG. 13 is a schematic of handpiece power supply system that may be usedwith one embodiment;

FIG. 14 illustrates non-zero Sense Power levels having durations thatare shorter than the Sense Power level durations shown in FIG. 11according to a further embodiment;

FIG. 15 illustrates non-zero Sense Power levels between cutting levelsand that have durations that vary over time according to a furtherembodiment;

FIG. 16 illustrates separate non-zero Sense Power pulses between cuttingpulses and power being zero between the cutting and Sense Power pulsesaccording to another embodiment;

FIG. 17 illustrates separate Sense Power pulses between cutting pulsesand the duration of the Sense Power pulses being shorter than theduration of the Sense Power pulses shown in FIG. 16 according to afurther embodiment;

FIG. 18 illustrates separate non-zero Sense Power pulses havingdurations that vary over time according to another embodiment;

FIG. 19 illustrates non-zero Sense Power pulses immediately prior tocutting pulses according to another embodiment;

FIG. 20 illustrates non-zero Sense Power pulses immediately followingcutting pulses according to another embodiment;

FIG. 21 illustrates separate Sense Power pulses between cutting pulsesand Sense Power measurements being made based on a decaying Sense Powerpulse according to another embodiment;

FIG. 22 illustrates measurements taken with respect to a slower decayingSense Power pulse;

FIG. 23 illustrates measurements taken with respect to a faster decayingSense Power pulse; and

FIG. 24 illustrates Sense measurements being taken with respect to arate of decay of a cutting pulse after the cutting pulse is switchedfrom a high level to a low level according to another embodiment.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

This specification describes embodiments of a method for controlling asurgical system for use in, for example, phacoemulsification surgery.Embodiments provide a surgical system that is able to detect stages of asurgical event, such as an occlusion, e.g., the onset of an occlusion, apre-occlusion condition, a full occlusion, and when an occlusion breaks,by detecting changes in the pressure levels of an aspiration system, anirrigation system, or a combination thereof. By monitoring how theaspiration or irrigation pressure levels vary, the onset and break up ofan occlusion can be accurately detected. Once an occlusion is detected,the surgical system can be programmed to increase the power available tothe handpiece, either by increasing the stroke of the tip or byincreasing the duty cycle of the pulsed ultrasound power.

To help avoid overheating of the tip, the surgical system monitors theirrigation flow rate and reduces the power to the handpieceautomatically if an overheating situation is predicted. Alternatively,or in addition, the amount of power drawn by the handpiece can bemonitored, which indicates the cutting load on the tip. This informationcan be used to automatically adjust the power or stroke (displacement)of the tip to compensate for load variations on the tip. In thefollowing description, reference is made to the accompanying drawings,which show by way of illustration, but not limitation, specificembodiments that can be utilized.

Embodiments can be implemented on commercially available surgicalsystems or consoles through appropriate hardware and software controls.One suitable system 100 is generally illustrated in FIG. 1 andrepresents the INFINITI® Vision System available from AlconLaboratories, Inc., 6201 South Freeway, Q-148, Fort Worth, Tex. 76134.

FIG. 2 illustrates one exemplary control system 100 in further detail.The control system 100 is used to operate an ultrasound handpiece 112and includes a control console 114, which has a control module or CPU116, an aspiration, vacuum or peristaltic pump 118, a handpiece powersupply 120, an irrigation pressure sensor 122 and a valve 124. Theconsole 114 may be any commercially available surgical control consolesuch as the ACCURUS® surgical system, also available from AlconLaboratories, Inc.

Various ultrasonic handpieces 112 and cutting tips can be utilizedincluding, but not limited to, handpieces and tips described in U.S.Pat. Nos. 3,589,363; 4,223,676; 4,246,902; 4,493,694; 4,515,583;4,589,415; 4,609,368; 4,869,715; 4,922,902; 4,989,583; 5,154,694 and5,359,996, the entire contents of which are incorporated herein byreference. Exemplary handpieces are shown in FIG. 3A-B for purposes ofexplanation, but not limitation.

Referring again to FIG. 2, the CPU 116 may be any suitablemicroprocessor, micro-controller, computer or digital logic controller.The pump 118 may be a peristaltic, a diaphragm, a Venturi or othersuitable pump. The power supply 120 may be any suitable ultrasounddriver, such as incorporated in the ACCURUS® surgical system, alsoavailable from Alcon Laboratories, Inc. The valve 124 may be anysuitable valve such as a solenoid-activated pinch valve. An infusion ofan irrigation fluid, such as saline, may be provided by a saline source126, which may be any commercially available irrigation solutionprovided in bottles or bags.

In use, the irrigation pressure sensor 122 is connected to the handpiece112 and the infusion fluid source 126 through irrigation lines 130, 132and 134. The irrigation pressure sensor 122 measures the pressure ofirrigation fluid from the source 126 to the handpiece 1-12 and suppliesthis information to the CPU 116 through the cable 136. The irrigationfluid pressure data may be used by the CPU 116 to control the operatingparameters of the console 114 using software commands. For example, theCPU 116 may, through a cable 140, vary the output of the power supply120 being sent to the handpiece 112 and the tip 113 though a power cable142. The CPU 116 may also use data supplied by the irrigation pressuresensor 122 to vary the operation of the pump 118 through a cable 144.The pump 118 aspirates fluid from the handpiece 112 through a line 146and into a collection container 128 through line 148. The CPU 116 mayalso use data supplied by the irrigation pressure sensor 122 and theapplied output of power supply 120 to provide audible tones to the user.Additional details concerning such surgical systems can be found in U.S.Pat. No. 6,179,808 (Boukhny, et al.) and U.S. Pat. No. 6,261,283(Morgan, et al.), the entire contents of which are incorporated hereinby reference.

In one embodiment, the control console 114 can control the amount ofpower that is delivered to the handpiece 112 based on the stage of anocclusion event. More particularly, power adjustments are made based onchanges of an aspiration vacuum level, an irrigation pressure level, orboth aspiration vacuum and irrigation pressure levels. The change canbe, for example, a rate of change of the increase or decrease ofaspiration vacuum and/or irrigation pressure.

Adjustments to the amount of power delivered to a handpiece can be madeas shown in FIG. 4. Initially, in step 400, a pattern of a surgicaloperating parameter during an occlusion or other surgical event isdetected over a period of time. The operating parameter can be a vacuumpressure and/or an irrigation pressure. Both pressures can also bedetected, however, reference is primarily made to a single operatingparameter for purposes of explanation, not limitation. In step 410, thevalues and/or the rate of change of the operating parameter can bedetermined or calculated. Based on this calculation, a stage of anocclusion is determined. In step 430, the amount of power that isdelivered to a cutting tip of the handpiece 112 can be adjusted, asnecessary, based on the stage of occlusion.

More specifically, it has been determined that aspiration vacuum andirrigation pressure levels follow a detectable pattern before, duringand after an occlusion. This pattern can be used to identify a stage ofan occlusion and adjust the power delivered to the handpiece 112accordingly.

As shown in FIG. 5, a typical occlusion event has the following stages:occlusion onset 500; pre-occlusion 510; full occlusion 520; occlusionbreak 530; and recovery 540. The term “onset” is generally used to referto the very beginning or preliminary stages of an occlusion, and“pre-occlusion” is generally used to refer to a time following anocclusion onset, and preceding full occlusion. In other words, “onset”is generally used to refer to the beginning of the development of anocclusion, and “pre-occlusion” is generally used to refer the stagewhere an occlusion is maturing to a full occlusion.

FIG. 6 illustrates in further detail patterns of aspiration vacuum andirrigation pressure that were detected. For each stage, the aspirationvacuum is shown as (mmHg) over time (t) and the pressure of anirrigation fluid or saline is shown as (cm H₂O) over the same time (t).These stages are discussed in further detail below.

As shown in FIG. 6, an occlusion onset event or condition 500 ischaracterized by a rapid increase 610 in the aspiration vacuum and arapid increase 615 in the irrigation pressure from a state ofnon-occlusion during which the vacuum and irrigation pressures arerelatively steady or constant (600 and 605). In other words, the ratesat which the vacuum and irrigation pressures are increasing are >0. Asshown, the onset 500 is identified by increasing aspiration vacuum andirrigation pressure. The irrigation pressure then may decrease slightly(617) and level off (618). The level of the aspiration vacuum, however,increases initially, and continues to increase while the irrigationpressure remains stable.

Following the occlusion onset event 500, the occlusion develops ormatures into a pre-occlusion event or condition 510. As shown in FIG. 6,a pre-occlusion event 510 is characterized by a slowing 620 of the rateof increase in aspiration vacuum, and a relatively stabilized irrigationpressure 625. Thus, the rate of increase of the aspiration vacuum andthe irrigation pressure both gradually decrease to a rate of zero. Inother words, both the vacuum and irrigation pressures become relativelystable.

The pre-occlusion condition 510 matures into a full occlusion 520. Afull occlusion is characterized by the maximum limit 630. Further, theirrigation pressure is steady 635.

Following the full occlusion 520, the occlusion breaks 530. An occlusionbreak event 530 is characterized by a rapid decrease of both theaspiration vacuum 640 and the irrigation pressure 645. As shown in FIG.6, both the aspiration vacuum and irrigation pressure levels rapidlydecrease (respective rates are <0) following a break of the occlusion.Following the rapid decrease, the rate of the decline of the aspirationvacuum and irrigation pressure level decrease 642, whereas theirrigation level pressure may reverse upward briefly 647 and thenstabilize 648.

Following the occlusion break 520 is an occlusion recovery stage 530. Arecovery stage 530 is characterized by a continued slowing of the rateof decrease of the aspiration vacuum 650 and irrigation pressure 655,eventually reaching a substantially constant level. In other words, therates of decline of the vacuum and irrigation pressures graduallyincrease from a negative value to approximately 0.

Based on the surgical systems tested, the patterns of vacuum andirrigation pressures shown in FIG. 6 are consistent from surgical systemto surgical system and can be detected using a variety of known digitalsignal processing methods. In one embodiment, the vacuum and irrigationpressures are detected using correlation methods. For example, phases ofan occlusion can be detected by calculating a linear correlation betweena pre-defined pattern and the actual aspiration vacuum or irrigationpressure sensor readings from the surgical system. The pre-definedpattern of aspiration vacuum defining occlusion onset can be, forexample, four points of the same vacuum reading followed by 12 points oflinearly increasing vacuum reading.

For example, the linear correlation between two sequences x_(i) andy_(i) is a measurement of how close one sequence can be transformed intothe other via a linear transformation:y _(i) =ax _(i) +bWhere: a=linear correlation coefficient, b=offset.

Given two sequences, the linear correlation R is calculated as follows:$R = \frac{{\sum\limits_{i = 0}^{N}\quad{x_{i}y_{i}}} - \frac{\sum\limits_{i = 0}^{N}\quad{x_{i}{\sum\limits_{i = 0}^{N}\quad y_{i}}}}{N}}{\sqrt{{\sum\limits_{i = 0}^{N}\quad x_{i}^{2}} - \frac{\left( {\sum\limits_{i = 0}^{N}\quad x_{i}} \right)^{2}}{N}}\sqrt{{\sum\limits_{i = 0}^{N}\quad y_{i}^{2}} - \frac{\left( {\sum\limits_{i = 0}^{N}\quad y_{i}} \right)^{2}}{N}}}$Where: N—correlation length (i.e. number of points in the sequences)

The linear correlation coefficient is calculated as follows:$a = \frac{{\sum\limits_{i = 0}^{N}\quad{x_{i}y_{i}}} - \frac{\sum\limits_{i = 0}^{N}\quad{x_{i}{\sum\limits_{i = 0}^{N}\quad y_{i}}}}{N}}{{\sum\limits_{i = 0}^{N}\quad x_{i}^{2}} - \frac{\left( {\sum\limits_{i = 0}^{N}\quad x_{i}} \right)^{2}}{N}}$

A method according to one embodiment involves calculating the linearcorrelation between a sample sequence of aspiration vacuum and/orirrigation pressure sensor readings collected during use of the surgicalsystem and the predefined pattern representing the occlusion events inquestion. The calculated correlation value reflects the similaritybetween the sample sequence and the predefined pattern, with the highestpossible value of 1.0 representing an absolute match. The range ofvalues indicating a sufficient correlation is preferably between 0.80and 0.99.

Once a match or acceptable correlation is established, the certainty ofthe some surgical events, such as pre-occlusion and occlusion recoveryis high, and the surgical parameters of the system can be adjusted asnecessary.

For events such as occlusion onset and occlusion break, the patternmatch should be qualified based on the rate of the change of the testvalues. The rate of change of vacuum and irrigation pressures can beevaluated using linear correlation coefficient, which reflects the sloperatio of the test sequence and the predefined pattern, and can thus beused to evaluate whether the sample sequence has a sufficient rate ofchange for a particular event.

In one embodiment, the rate of change is a direct calculation of thederivative (ΔValue/ΔTime), or the change in a value over a certain time.The criteria for a sufficient rate can be established empirically for agiven system at different settings (e.g. different aspiration pumprates).

For cases that require qualification on both pattern match and the rateof change, the occlusion event is considered to be detected when bothconditions are satisfied. Once the occlusion event is detected thesurgical parameters of the system can be adjusted. The described methodcan be applied to detecting all events in an occlusion sequence(occlusion onset, pre-occlusion, occlusion, occlusion break, andrecovery). By detecting patterns of aspiration vacuum and/or irrigationpressure levels, the timing of when power should be adjusted can beaccurately determined. Thus, embodiments are more accurate than knownsystems that rely on a threshold or pre-determined aspiration vacuumlevels to identify a full occlusion.

In a further embodiment, the matching of patterns can be accomplishedusing convolution rather than correlation. Accordingly, persons ofordinary skill in the art will appreciate that the correlation,derivative, and convolution techniques discussed herein are merelyillustrative examples, and are not intended to be limiting.

In a further embodiment, the amount of power delivered to the handpiececan be regulated to prevent overheating of the tip 113, which can leadto damage to the eye tissue. This embodiment is referred to as ThermalWatch™ and is generally illustrated in FIG. 7.

In step 700, a threshold temperature is established. In step 710, apressure of a source of an irrigation fluid 126, such as saline, ismonitored. In step 720, a calculation or determination is made of theflow rate of irrigation fluid from the irrigation fluid source 126 beingmonitored. A capacity of the flow of irrigation fluid to absorb heat, orthe heat absorption capacity of the irrigation fluid, is determined instep 730. In step 740, the determined heat absorption capacity and thepower supplied to the handpiece 112 are compared or analyzed. Based onthis comparison or analysis, a temperature of the eye or other tissue isdetermined in step 750.

For example, an approximate temperature of the eye can be determined byperforming a temperature calculation at discrete time steps where theestimated temperature at the current time is found by multiplying theprevious temperature estimation by a cooling coefficient (which is <1)then adding the power delivered during the time interval times a powercoefficient and subtracting the flow during the time interval times aflow coefficient.

In step 760, the estimated eye temperature and the threshold temperatureare compared. The amount of power delivered to the cutting tip 113 ofthe ultrasound handpiece 112 is adjusted, as necessary, in step 770,based on whether the estimated temperature exceeds the threshold orexceeds the threshold by a predetermined amount. For example, the powerdelivered to the handpiece may be reduced by reducing amplitude of theultrasound signal and/or decreasing duty cycle of the ultrasound signal,if the estimated temperature exceeds the threshold, whereas the powermay be maintained or increased if the estimated temperature is below thethreshold.

FIG. 8 illustrates one implementation of the process shown in FIG. 7.Referring to FIG. 8, in step 800, a determination is made whether theThermal Watch™ feature is enabled. If Thermal Watch™ is not enabled,then in step 805, the system operates using linear ultrasound controlfunctions. In other words, the ultrasound power delivered is controlledby the console settings and the surgeon's depression of the foot pedal.

If Thermal Watch™ is enabled, then in step 810, a threshold value, setby the surgeon, is noted or read by the system. The threshold value maybe unitless and be in any number of steps from “Off” to “Maximum”.

In step 815, the system monitors the pressure of the irrigation fluid(the “IPS Reading”) and/or the height of the irrigation fluid sourcebottle 126 in step 820. These irrigation fluid pressure parameters,indicate the rate of flow of irrigation fluid, i.e., the quantity ofirrigation fluid over a certain time, in step 825. Knowing the rate ofirrigation fluid flow, the heat quenching capacity for that rate ofirrigation fluid flow can be approximated (step 830). The flow functionin time F(t) can approximated a linear function of pressure drop acrossthe fluidics line: F(t)=R(P₀−P(t)), where P₀ is the irrigation sourcepressure (e.g. irrigation bottle height), P(t) is irrigation pressuresensor reading, and R is fluidic line resistance between the pressuresource and the irrigation pressure sensor. The resistance R isestablished empirically for a given fluidic (i.e. consumable)configuration. The above approximation yields accurate results for asteady state flow conditions. To improve estimation accuracy fortransient response an exponentially decaying correction can by added tothe equation above as follows:${F(t)} = {R\left\lbrack {{\left( {1 + \delta} \right)\left( {P_{0} - {P(t)}} \right)} - {\delta\frac{1}{\tau_{0}}{\int_{- \infty}^{t}{{{\mathbb{e}}^{- \frac{\tau}{\tau_{0}}}\quad\left( {P_{0} - {P(\tau)}} \right)}{\mathbb{d}\tau}}}}} \right\rbrack}$where δ is the transient coefficient, and τ₀ is the time constant of theapproximated fluidic line. Both values can be established empiricallyfor a given fluidic (i.e. consumable configuration). Sample valuesestablished for the Alcon INFINITI® system consumable are: δ=0.3, τ₀=1.3seconds. The equation above can easily converted into a discrete formallowing practical implementation of the method.

The amount of heat that is generated by the ultrasonic cutting tip 113of the handpiece 112 (i.e., the Ultrasonic or “US Power Level”) is alsomonitored in step 835. The approximation of the heat quenching capacityfor the irrigation fluid flow is then compared to the amount of heatthat is generated by the ultrasonic cutting tip 113 to determine anapproximate temperature of the eye in step 840. A determination is madewhether the temperature of the eye is higher than a preset thresholdvalue or is within a certain margin of the threshold in step 845. Forexample, the margin may be three degrees Fahrenheit (3° F.) within (e.g.below) the threshold, as shown in FIG. 8. Persons skilled in the artwill appreciate that other pre-determined amounts or margins can also beutilized depending on the desired sensitivity of the system.

If the temperature of the eye is not within the margin (e.g. 3° F.) ordoes not exceed the threshold, then linear ultrasound control functionscan be used (step 850). However, if the temperature of the eye is withinthe margin or exceeds the threshold, then the system utilizes analgorithm to compute an appropriate duty cycle in step 855. The controlalgorithm may be, for example, a standard linear control algorithm suchas a PI (proportional-integral) or PID(proportional-integral-derivative) control algorithm. The controlalgorithm may also be a non-linear control algorithm, such as abang-bang controller with or without hysteresis. Persons skilled in theart will appreciate that various algorithms can be used in differentapplications.

For example, in step 860, a determination is initially made whether thesystem currently operates in a continuous mode. In continuous mode, acontinuous supply of power is applied to the handpiece withoutinterruption. If the system is in continuous mode then in step 865, thesystem switches the mode of operation from continuous to pulsedultrasonic power. If the system is not in continuous mode, then adetermination is made in step 870 whether the system is operating inburst or pulse mode.

Referring to FIGS. 9 a-9 b, burst mode provides a series of periodic,fixed width, constant amplitude pulses 900 of ultrasound power, each ofwhich is followed by an “off” time 910. Persons skilled in the art willappreciate that in practice, the pulses shown in FIGS. 9 a-9 b and otherfigure are not “perfect” pulses. Rather, the pulses transition or rampbetween different states due to, for example, capacitance andinductance. Thus, the ideal or model rectangular pulses shown in FIGS. 9a-9 b and other figures are provided for purposes of explanation andillustration when, in practice, the pulses do not have a perfectrectangular shape.

The off time 910 between fixed width pulses 900 is controlled by thesurgeon's input via, for example, depression of the foot pedal. In otherwords, in Burst mode, each pulse 900 has a fixed “on” time, and avariable “off” time. The “off” time is varied by adjusting the positionof the foot pedal or foot switch.

For example, FIGS. 9 a-9 b illustrate a foot switch in four positions:The off time 910 decreases when the foot pedal is initially at Position1 and depressed further to Position 2, and decreases further when thefoot pedal is depressed from Position 2 to Position 3. Thus, the numberof fixed width, constant amplitude pulses in a period of time increaseas the foot pedal is depressed further. As the foot pedal is depressedfurther, the off time eventually equals the on time. In this case,further depression of the foot pedal from position 3 to position 4results in the amplitude of the pulses being increased, whilemaintaining the same off time 910. In other words, pulse amplitude canbe made after the off time is the same as the on time, therebyincreasing power delivered to the handpiece.

Referring again to FIGS. 8 and 9 a-9 b, if the system is in burst mode,a number of pulses of ultrasound power having the same pulse width canbe adjusted in step 875 by decreasing the power “On” time or increasingthe power “Off” time, thereby providing fewer fixed width pulses in agiven time and less power to the ultrasonic tip 113, in order to coolthe tip 113. If the system is not in burst mode, in step 880, the systemis in a pulse mode.

Referring to FIGS. 8 and 10, if the system is in pulse mode, then theamplitude of the pulses 1000 remains constant, and the power deliveredto the handpiece can be adjusted in step 885 by adjusting the duty cycleof the power pulses being supplied to handpiece 112. In an ideal trainof rectangular pulses 1000, the ratio of the pulse duration to the pulseperiod or the ratio of the duration of the “high” pulse level to the sumof the durations of the “high” and “low” levels (one period) is the dutycycle, represented as a fraction or percentage. Thus, the duration ofeach constant amplitude pulse 1000 can be changed (e.g., become narroweror wider) to change the duty cycle, and thus change the amount of powerthat is delivered to the handpiece.

Alternatively, if the system is operating in continuous mode (860), andthe temperature of the eye is above the threshold, then the power can beswitched off until the eye temperature drops below the threshold.Further, if the system is operating in a burst or pulse mode andtemperature of the eye is above the threshold, power can be turned offfor a remainder of a pulse, and the next power pulse can be delayed, ifnecessary, until the eye temperature drops below the threshold.

Persons skilled in the art will appreciate that the Thermal Watchfeature can be used with different types of pulses, e.g., continuous,pulsed, burst, and pulses having different patterns, such as pulsesdescribed later in this specification and shown in FIGS. 14-24 sinceThermal Watch serves as a governor that periodically determines theamount of power delivered, regardless of the type of pulse or pulsepattern, and how that determined amount of power compares to thethreshold and how the amount of power compares to the threshold, aspreviously described.

Similar power adjustments can be made when an occlusion event isdetected. For example, in one embodiment, the power delivered to the tip113 can be increased by increasing the “On” time or by decreasing thepower “Off” time in the ultrasound duty cycle so as to increase thecutting efficiency of handpiece 112. The temperature of eye can bemonitored using the Thermal Watch™ feature to decrease the power “On”time, or increase the power “Off” time prior to tip 113 becomingoverheated. Accordingly, embodiments provide a manner of effectivelyincreasing power when necessary (e.g., when an occlusion event occurs),but effectively monitoring and reducing the power, as necessary, inorder to Is prevent overheating of the tip 113 and burning or damagingeye tissue.

An alternative embodiment is directed to a load detection system andmethod, generally referred to as “Power on Demand,” can limit oroverrule the amount of power that is requested by a surgeon if it isdetermined that too much power is being requested in order to preventdamage to eye tissue. The system can detect when the cutting tip 113 isno longer in contact with lens tissue or is in contact with differentsections of a lens tissue with varying hardness, and automaticallyadjusts the amount of power delivered to the handpiece.

As previously discussed, one or more piezoelectric crystals in thehandpiece 112 are driven by power that is provided by a power supply.The crystals vibrate, which in turn causes a needle in the handpiece tovibrate and emit ultrasound energy. The surgeon positions the needle sothat the ultrasound energy is delivered to an eye tissue, such as a lensto, for example, break up cataracts. A separate aspiration system isused to remove tissue fragments. A surgeon may request that a certainamount of power be delivered to the tip 113, by, for example, depressinga foot switch or other switch activator. During the surgical procedure,the system applies a low level of voltage to the crystals causing themto emit a small amount of ultrasound energy. The voltage across thecrystals and the current through the crystals under this condition arethen measured. The voltage and current values are used to calculate apower value that is drawn by the handpiece. The ultrasound handpiece 112tends to draw more power in order to maintain a given stroke (amplitude)when the tip 113 is in contact with harder tissue or material (such as acataractuous lens). This increase in power based on contact withmaterial encountered by the tip in a typical cataract surgery has beenfound to be measurable at lower power settings. In a modified pulsedmode, a small amount of power is applied to the tip 112 between thehigher power pulses used to cut the tissue. In other words, a smallamount of power is applied during low power periods.

For example, as shown in FIG. 11, the pulsed mode type driving signalincludes alternating intervals of high or cutting power 1100 a-e(generally 1100) and alternating intervals of low or sense power 1110a-e (generally 1110) between the cutting intervals 1100. The amplitudeof the sense interval 1110, however, is greater than zero. In otherwords, the sense power does not decrease to zero following a cuttinginterval.

In the illustrated embodiment, the duration of the cutting interval 1100and sense interval 1110 are approximately the same. Voltage and currentmeasurements are performed during the sense intervals in order tocorrelate an amount of power that is drawn by the handpiece 112 duringthe sense interval, with a load 1130 at the tip 113. Some degree ofcutting may also occur since a small amount of power is still applied tothe tip, however, cutting primarily occurs during the higher powercutting interval. Thus, although this specification refers to a “sense”interval, both sensing and cutting may occur during this interval.

The amount of power drawn by a handpiece 112 is determined during thesense interval 1110 is then used to adjust the power of the next orsubsequent cutting interval 1100. The power is adjusted proportionatelybased on the detected power and the surgeon's request. In other words,if a higher load is detected at the tip, a higher in portion (possiblyall) of the power requested by a surgeon will be delivered on the nextcutting interval. Likewise, if a lower load is detected, a smallerportion (possibly none) of the power requested by the surgeon will bedelivered during the next cutting interval 1110.

For example, the power detected during sense interval 1110 a is used toproportionately adjust the power level of the next cutting interval 1100b. Similarly, the power detected during sense interval 1110 b is used toproportionately adjust the next cutting interval 1100 c. Thus, thecutting power 1100 is continuously adjusted to compensate for differentloads 1130 on the ultrasonic tip 113 of the handpiece 112.

As shown in FIG. 11, the power level of the sense interval 1110 isrelatively constant over time. The sense interval 1110, however, mayvary, but should not be zero or so low that a load at the tip cannot bedetected. The power level of the sense interval 1110 can vary dependingon, for example, system parameters and the sensitivity of measuringequipment. Accordingly, embodiments using non-zero sense periods are incontrast to known “pulse mode” driving systems that typically usealternating high power and zero power pulses, i.e., switching between“on” and “off” rather than switching between high power and low power or“on” and “low power.”

Due to the variation in ultrasound handpieces and cutting tips, the load1130 sensing feature should be calibrated at the beginning of eachsurgery. For example, calibration can be performed during a “tune cycle”during which the handpiece tip 113 is placed in a test chamber filledwith irrigation fluid. At this time, ultrasound power is applied to thetip 113 at the sensing power setting. The amount of power drawn by thehandpiece 112 under this baseline condition is saved in the controlsystem memory as a threshold or a “no-load” condition. If necessary, assurgery progresses, the control system 114 may use automatic thresholdadjustment to adjust this threshold value based on loads 1130 that aremeasured during the surgery.

The load sensing feature also allows the surgeon to control thesensitivity of the adjustments made by the control system 114. Morespecifically, a sensitivity adjustment is a linear or coefficientadjustment that sets the gain of the power reductions made when lessthan full load is sensed. Once the threshold and sensitivity are set,the power to the handpiece 112 may be adjusted based on an algorithm.

FIGS. 12 and 13 illustrate one embodiment of an algorithm and systemthat operates based on the algorithm for performing these power strokeor amplitude variations based on the sensed voltage and current load1130. Initially, a threshold value 1355 is determined in step 1200. Aspreviously discussed, the threshold power 1355 is a fixed value that isdetermined after operating the ultrasonic handpiece 112 in an irrigationsolution or saline or other reference environment. In other words, thethreshold power 1355 represents a baseline power when no tissue is beingcut.

The power drawn by the cutting tip 113 is monitored in step 1210. Apower supply 120 is coupled to the handpiece 112 and delivers power tothe tip 113 via a power line 1302. A power control system 1300 isconnected to the inputs and outputs of the power supply 120 viaconnectors 1303-1305. The connectors 1304 and 1305 are coupled to theoutput of the power supply 120, and the connector 1303 is coupled to theinput of the power supply 120.

The power control system 1300 shown includes feedbacks 1310 and 1311 forvoltage and current. The feedback components 1310 and 1311 are coupledto the connectors 1304 and 1305. Voltage and current measurementsobtained during sense intervals 1110 are based on the stresses placed onthe piezoelectric crystals in the handpiece 112. If the needle or tip113 encounters tissue that is more resistant to cutting (a high load),the magnitudes of the voltage and/or the current may increase, whereasif the needle or tip 113 encounters tissue that is more easily cut (alower load), the magnitudes of the voltage and/or current may decrease.The voltage and current values obtained by the feedbacks 1310 and 1311during each sense interval 1110 are provided to respective Root MeanSquare (“RMS”) converters 1320 and 1321.

The RMS converters 1320 and 1321 determine an average voltage magnitudeand an average current magnitude over a defined period of time. TheseRMS values are provided to Analog to Digital (“A/D”) converters 1330 and1331, which provide digital signals representing the respective measuredanalog voltage and current to a microprocessor 1340.

The microprocessor 1340 can be the same microprocessor 116 as discussedabove or a separate microprocessor or controller. The digital voltageand current values are provided by the A/D converters 1330 and 1331 tothe microprocessor 1340. Software in the microprocessor 1340 calculates1350 the “Sense” power 1352 detected during a sense interval 1110 asPower (“P”)=Voltage (“V”)×Current (“I”) based on the values provided bythe A/D converters 1330 and 1331. Thus, the calculation involves alinear calculation without the necessity to account for non-linearattributes, such as phase and resonance. The sense power 1352 is thencompared to the threshold or baseline power 1355.

The calculated sense power 1352 exceeds the threshold or base power 1355when power is needed by the handpiece 112 to cut tissue, i.e., when thehandpiece 112 is applied to something other than the base material orliquid, such as saline. The comparison of sense power 1352 and thethreshold power 1355 is used to determine how the power to be deliveredto the handpiece 112 should be adjusted, if at all, during the nextcutting interval in step 1240 based on the characteristics of the tissueencountered by the tip 113 of the handpiece 112. This comparison ismultiplied by a scaling coefficient 1356 that is stored in the handpieceor in the software that relates the amount by which the sense power 1352exceeds the threshold power 1355 to the fraction of full loadingdetected 1357. The scaling coefficient 1356 can, for example, beempirically determined based on prior operation of the system.

In addition to this threshold comparison and percent load calculation, asensitivity adjustment or coefficient 1360 is set by the surgeon toindicate what fraction of the power requested by the surgeon should bedelivered to the tip during the next cutting interval based on an amountby which the sense power 1352 exceeds the threshold power 1355. Thesensitivity coefficient 1360 ranges from 0-1 or is otherwise generallyrepresented as a % value, e.g., 20%, 50% or 85%. These values may berepresented to the surgeon as off, low, medium, high or some other scaleor indication. In step 1250, values obtained by the (sensevoltage−threshold)×scaling factor calculation are multiplied by thesensitivity coefficient 1360. A greater quantity of the requested power1370 (e.g., as indicated by the level of a footswitch 1375) is deliveredto the handpiece 112 with higher sensitivity coefficients than withlower sensitivity coefficients. For example, if the surgeon requests “X”amount of power 1370 via the foot pedal 1375, then some, all, or none ofthat “X” power 1370 may be delivered to the handpiece 112 depending onthe sensitivity coefficient 1360.

Thus, the power 1380 that is actually delivered to the handpiece 112 maybe less than or the same as the amount of power requested 1370 by asurgeon by depression of the foot pedal 1375. Accordingly, theembodiments use linear relationships and calculations, thresholddeterminations and linear calculations based on sensitivity coefficientsin order to adjust the amount of power 1380 that is delivered to ahandpiece 112.

FIG. 11 illustrates one pulse pattern that includes cutting and senseintervals for use with the Power on Demand system shown in FIGS. 12 and13. The pulse pattern shown in FIG. 11 includes cutting and senseintervals that are relatively constant and approximately the sameduration. In alternative embodiments, different pulse patterns may beused with different cutting and sense intervals, as shown in FIGS.14-24. In order to illustrate the different cutting and sensing pulsesand intervals, the pulses are shown without a corresponding load,however, persons skilled in the art will appreciate that the amplitudesof the cutting intervals may be adjusted as necessary depending on theload at the tip of the handpiece. This specification refers to an“interval” and a “pulse”. A pulse is a signal that begins from and endsat zero power, whereas an interval can be considered to be part of apulse and thus, either begins or ends at zero power. However, forpurposes of this specification, these terms are interchangeable sincethey both provide durations of sense power and durations of cuttingpower. Accordingly, “interval” is intended to include a “pulse” and a“pulse” is intended to include an “interval”.

Referring to FIG. 14, in an alternative embodiment, the durations of thesense intervals 1110 are approximately the same over time and shorterthan the durations of the cutting intervals 1100. In a furtherembodiment, shown in FIG. 15, the duration of the sense intervals 1110can vary over time so that they are shorter than, about the sameduration as, or longer than the cutting intervals. The duration of senseintervals 1110 can be adjusted to accommodate, for example, Signal toNoise (S/N) ratios and system response. A longer sense interval 1110 mayprovide better S/N ratios and a more delayed response. Thus, theduration of sense intervals 1110 can be selected to coordinate withsystem components and capabilities.

Referring to FIG. 16, in a further alternative embodiment, the senseinterval 1110 can immediately precede separate cutting interval 1100.For example, power increases from a zero level to a low power levelduring the sense interval 1110. Immediately following the sense interval1100 is the cutting interval 1100. The cutting interval 1100 is at ahigher power level than the sense interval 1110. After the cuttinginterval 1100, the power returns to zero, and the interval sequence canbe repeated. FIG. 17 illustrates a similar configuration except that thehigh power cutting pulse 1100 immediately follows a period of zeropower. The sense interval 1110 immediately follows the higher powercutting interval 1100 and then followed by zero power, which can berepeated as necessary.

FIG. 18 illustrates another embodiment in which a separate low power,sense pulses 110 are triggered between separate higher power cuttingpulses 1100. In the illustrated embodiment, the cutting and sense pulses1100 and 1110 have about the same duration. FIG. 19 illustrates anotheralternative embodiment that utilizes separate sense pulses 1110 andcutting pulses 1100, and in which the duration of the sense pulse 1110is shorter than the duration of the cutting pulse 1100. FIG. 20illustrates a further embodiment in which separate sense pulses 1110have varying durations and are between cutting pulses 1100.

FIG. 21 illustrates yet a further alternative embodiment that includesseparate sense pulses 1110 and cutting pulses 1100, and where thevoltage and current data are obtained during the decay 2100 of a sensepulse 1110. This embodiment is illustrated in further detail in FIGS. 22and 23. Rather than determining the load as shown in FIG. 11, the systemcan be configured to determine the time that it takes for a senseinterval pulse 1110 to decay to a certain level. The rate of decay canbe affected by the load on the tip. For example, a larger load willcause the sense pulse to decay more quickly, whereas a smaller load willresult in the sense pulse decaying more slowly. FIG. 22 shows a sensepulse taking longer to decay due to a smaller load, and FIG. 23illustrates a sense pulse decaying more quickly, due to a larger load.This decay technique can also be applied to other pulse patterns,including sense intervals that immediately follow a cutting interval,such as sense intervals shown in FIG. 17.

The time required for a sense pulse or interval to decay to a certainlevel can be correlated to a load at the tip. This can be done using,for example, a lookup table that cross-references the rate of decay anda load at the tip. This decay and load information can then be used toadjust the power level of the next cutting pulse or interval ifnecessary. For example, referring to reference Point C in FIGS. 22 and23, the rate of decay of the pulse shown in FIG. 23 is faster than therate of decay of the pulse shown in FIG. 22. As a result, the amount ofpower delivered during the next cutting pulse following the sense pulseshown in FIG. 22 may be less than amount of power delivered during thenext cutting pulse following the sense pulse shown in FIG. 23 since thepulse shown in FIG. 23 decays faster due to a larger load at the tip.The rate of decay analysis can be repeated to continuously adjust thepower delivered to the tip during the next cutting pulse or interval

According to a further embodiment, cutting and sense pulses can be atdifferent frequencies. For example, cutting pulses can be applied at aparticular frequency, and a sense pulses can be applied at one of theharmonics of the cutting pulse frequency. For example, cutting pulsesmay be applied at about 40 kHz, and sense pulses can be applied at oneof the harmonics, such as 80 kHz or 120 kHz.

Persons skilled in the art will appreciate that the FIGS. 11 and 14-23are provided as exemplary sense and cutting interval patterns and arenot intended to be limiting since sense and cutting intervals can beadjusted as necessary for different systems and applications. Further,persons skilled in the art will appreciate that both sensing and somedegree of cutting may occur during a lower power sense interval sincesensing occurs at a non-zero level, and some cutting occurs, althoughthe amount of cutting is small compared to cutting that occurs during ahigher power cutting interval. Persons skilled in the art will alsoappreciate that the Thermal Watch feature can be used with thesedifferent pulse patterns since the Thermal Watch considers the amount ofpower delivered and is not dependent on a particular pulse pattern.

Referring to FIG. 24, in a further alternative embodiment, the rate ofdecay 2400 of a cutting pulse 1100 can be correlated to a load at thetip. Depending on the amplitude of the cutting pulse 1100, it may bedesirable to sample the tail end 2410 of the decaying pulse 2400 sincethe power level of the decaying cutting pulse may be too high at thebeginning of the decay period, thereby causing interference with thepower and current measurements. The time required for a cutting pulse todecay to a certain level can be cross-referenced with a lookup table sothat the rate of decay can be correlated to a load at the tip. Thisdecay and load information can then be used to adjust the power level ofthe next cutting pulse if necessary.

Although references have been made in the foregoing description tovarious embodiments, persons of ordinary skill in the art will recognizethat insubstantial modifications, alterations, and substitutions can bemade to the described embodiments without departing from the scope ofembodiments. For example, persons of ordinary skill in the art willrecognize that various capabilities and embodiments can be operatedindividually or in combination. For example, in an alternativeembodiment, the embodiments directed to determining changes inaspiration vacuum and/or irrigation pressure can be combined with the“Thermal Watch” embodiments shown in FIGS. 7 and 8 and/or with the“Power On Demand” embodiments described and shown with reference toFIGS. 9-11. Similarly, the “Thermal Watch” embodiments described andshown with reference to FIGS. 7 and 8 can be combined with the Power OnDemand embodiments described and shown with reference to FIGS. 9-11.Thus, embodiments can operate together or individually to provide thedesired system functionality.

1. A method of controlling a surgical system, the surgical system having an ultrasound handpiece, the ultrasound handpiece having a cutting tip, the method comprising the steps of: establishing a threshold temperature; monitoring an irrigation pressure in a line running to the ultrasound handpiece; calculating a rate of flow of the irrigation fluid; determining a heat absorption capacity for the calculated rate of flow of irrigation fluid; determining a temperature of an eye based on the determined heat absorption capacity and an amount of power being supplied to the ultrasound handpiece; comparing the determined temperature of the eye to the threshold temperature; and if necessary, adjusting the amount of power delivered to the cutting tip of the ultrasound handpiece based on the comparison of the determined and threshold temperatures.
 2. The method of claim 1, adjusting the amount of power comprising adjusting a duty cycle of the output of the ultrasound handpiece.
 3. The method of claim 1, adjusting power comprising adjusting a stroke or amplitude of the output of the ultrasound handpiece.
 4. The method of claim 1, adjusting power comprising reducing power if the determined temperature exceeds the threshold temperature.
 5. The method of claim 1, wherein the power is maintained or increased if the determined temperature is below the threshold temperature.
 6. A method of controlling a surgical system, the surgical system having an ultrasound handpiece, the ultrasound handpiece having a cutting tip, the method comprising the steps of: establishing a threshold temperature; monitoring an irrigation pressure in a line running to the ultrasound handpiece; calculating a rate of flow of the irrigation fluid; determining a heat absorption capacity for the calculated rate of flow of irrigation fluid; determining a temperature of an eye based on the determined heat absorption capacity and an amount of power being supplied to the ultrasound handpiece; comparing the determined temperature of the eye to the threshold temperature; and determining a rate of change of the irrigation pressure; determining a stage of occlusion based on the rate of change of the irrigation fluid; and if necessary, adjusting an amount of power delivered to the cutting tip of the ultrasound handpiece based on the determined stage of occlusion and based on the comparison of the determined and threshold temperatures.
 7. The method of claim 6, adjusting the amount of power comprising adjusting a duty cycle of the output of the ultrasound handpiece.
 8. The method of claim 6, adjusting power comprising adjusting a stroke or amplitude of the output of the ultrasound handpiece.
 9. The method of claim 6, adjusting power comprising reducing power if the determined temperature exceeds the threshold temperature.
 10. The method of claim 6, wherein the power is maintained or increased if the determined temperature is below the threshold temperature.
 11. The method of claim 6, further comprising detecting a pattern of data of the irrigation pressure.
 12. The method of claim 11, detecting the pattern comprising determining a linear correlation between a predefined pattern and data of the irrigation pressure during operation of the surgical system.
 13. The method of claim 12, determining the linear correlation comprising performing a linear transformation.
 14. The method of claim 11, detecting the pattern comprising calculating a derivative of data of the first operating parameter over time.
 15. The method of claim 11, detecting the pattern comprising performing a convolution of a predefined pattern and data of the first operating parameter during operation of the surgical system.
 16. The method of claim 1, further comprising determining a rate of change of aspiration vacuum of the ultrasound handpiece.
 17. The method of claim 16, wherein determining the stage of occlusion is based on the rate of change of the irrigation pressure and aspiration vacuum.
 18. The method of claim 6, determining the stage of occlusion comprising identifying an onset or preliminary indication of occlusion.
 19. The method of claim 18, the onset being identified by an increasing aspiration vacuum.
 20. The method of claim 19, further comprising determining a rate of change of aspiration vacuum, the onset being identified by increasing irrigation pressure and aspiration vacuum.
 21. The method of claim 20, the irrigation pressure increasing at a faster rate than the aspiration vacuum.
 22. The method of claim 6, determining the stage of occlusion comprising identifying a pre-occlusion condition.
 23. The method of claim 22, the pre-occlusion condition being identified by an a substantially constant irrigation pressure
 24. The method of claim 19, further comprising determining a rate of change of an aspiration vacuum, the pre-occlusion condition being identified by a substantially constant irrigation pressure and an increasing aspiration vacuum.
 25. The method of claim 24, the aspiration vacuum during the pre-occlusion condition increasing at a slower rate than during a period of time between an onset of occlusion and the pre-occlusion condition.
 26. The method of claim 6, determining the stage of occlusion comprising identifying a full occlusion.
 27. The method of claim 26, further comprising determining a rate of change of an aspiration vacuum, the full occlusion being identified by substantially constant irrigation pressure and aspiration vacuum.
 28. The method of claim 6, determining the stage of occlusion comprising identifying a break of an occlusion.
 29. The method of claim 28, the first operating parameter comprising an irrigation pressure, the break being identified by a decreasing irrigation pressure.
 30. The method of claim 29, further comprising determining a rate of change of an aspiration vacuum, the break of an occlusion being identified by decreasing irrigation pressure and aspiration vacuum.
 31. The method of claim 6, determining the stage of occlusion comprising identifying a recovery from a break of an occlusion.
 32. The method of claim 31, the recovery being identified by a slowing of a rate of decrease of the irrigation pressure.
 33. The method of claim 32, further comprising determining a rate of change of an aspiration vacuum, the recovery being identified by a slowing of a rate of decrease of the irrigation pressure and a slowing of a rate of decrease of the aspiration vacuum.
 34. The method of claim 6, determining the stage of occlusion comprising determining identifying an onset of an occlusion, a pre-occlusion condition following the onset, a full occlusion following the pre-occlusion condition, a break of the full occlusion; or a recovery following the break.
 35. A method of controlling a surgical system, the surgical system having an ultrasound handpiece, the ultrasound handpiece having a cutting tip, the method comprising the steps of: establishing a threshold temperature; monitoring an irrigation pressure in a line running to ultrasound handpiece; calculating a rate of flow of the irrigation fluid; determining a heat absorption capacity for the calculated rate of flow of irrigation fluid; determining a temperature of an eye based on the determined heat absorption capacity and an amount of power being supplied to the ultrasound handpiece; comparing the determined temperature of the eye to the threshold temperature; and determining a rate of change of the irrigation pressure; determining a stage of occlusion based on the rate of change of the irrigation fluid; and if necessary, adjusting an amount of power delivered to the cutting tip of the ultrasound handpiece based on the determined stage of occlusion and based on the comparison of the determined and threshold temperatures., the power delivered to the handpiece being reduced if the determined temperature exceeds the threshold temperature, the power delivered to the handpiece being maintained or increased if the determined temperature is below the threshold temperature.
 36. The method of claim 35, adjusting the amount of power comprising adjusting a duty cycle of the output of the ultrasound handpiece.
 37. The method of claim 35, adjusting power comprising adjusting a stroke or amplitude of the output of the ultrasound handpiece.
 38. The method of claim 35, further comprising detecting a pattern of data of the irrigation pressure.
 39. The method of claim 38, detecting the pattern comprising determining a linear correlation between a predefined pattern and data of the irrigation pressure during operation of the surgical system.
 40. The method of claim 39, determining the linear correlation comprising performing a linear transformation.
 41. The method of claim 38, detecting the pattern comprising calculating a derivative of data of the irrigation pressure over time.
 42. The method of claim 35, detecting the pattern comprising performing a convolution of a predefined pattern and data of the irrigation pressure during operation of the surgical system.
 43. The method of claim 35, further comprising determining a rate of change of an aspiration vacuum.
 44. The method of claim 43, wherein determining the stage of occlusion is based on the rate of change of the irrigation pressure and aspiration vacuum.
 45. The method of claim 35, determining the stage of occlusion comprising identifying an onset or preliminary indication of occlusion.
 46. The method of claim 45, the onset being identified by an increasing aspiration vacuum.
 47. The method of claim 35, determining the stage of occlusion comprising identifying a pre-occlusion condition.
 48. The method of claim 47, the pre-occlusion condition being identified by a substantially constant irrigation pressure
 49. The method of claim 48, further comprising determining a rate of change of an aspiration vacuum, the pre-occlusion condition being identified by a substantially constant irrigation pressure and an increasing aspiration vacuum.
 50. The method of claim 49, the vacuum pressure during the pre-occlusion condition increasing at a slower rate than during a period of time between an onset of occlusion and the pre-occlusion condition.
 51. The method of claim 35, determining the stage of occlusion comprising identifying a full occlusion.
 52. The method of claim 51, further comprising determining a rate of change of an aspiration vacuum, the full occlusion being identified by substantially constant irrigation pressure and aspiration vacuum.
 53. The method of claim 35, determining the stage of occlusion comprising identifying a break of an occlusion.
 54. The method of claim 53, the first operating parameter comprising an irrigation pressure, the break being identified by a decreasing irrigation pressure.
 55. The method of claim 54, further comprising determining a rate of change of an aspiration vacuum, the break of an occlusion being identified by decreasing irrigation pressure and aspiration vacuum.
 56. The method of claim 35, determining the stage of occlusion comprising identifying a recovery from a break of an occlusion.
 57. The method of claim 56, the recovery being identified by a slowing of a rate of decrease of the irrigation pressure.
 58. The method of claim 57, further comprising determining a rate of change of an aspiration vacuum, the recovery being identified by a slowing of a rate of decrease of the irrigation pressure and a slowing of a rate of decrease of the aspiration vacuum.
 59. The method of claim 35, determining the stage of occlusion comprising determining identifying an onset of an occlusion, a pre-occlusion condition following the onset, a full occlusion following the pre-occlusion condition, a break of the full occlusion; or a recovery following the break. 